Printing systems including a plurality of printing modules, also referred to as marking modules, are known and can be generally referred to as tandem engine printers. Such systems especially facilitate expeditious duplex printing, i.e., printing on both sides of a media sheet or document, with the first side of a document being printed by one of the printing modules and the opposite, or second side, of the document being printed by a second printing module. The process path for the document usually requires an inversion of the document to facilitate printing on the second side of the document.
Media sheet inverters are well known and essentially comprise an arrangement of nip wheels or rollers which receive a document by extracting it from a main process path, then direct it back onto the process path after a 180 degree flip so that what had been the trailing edge of the document, now leaves the inverter as the leading edge along the main process path.
Inverters are thus fairly simple in their functional result; however, complexities occur as the printing system is required to handle different sizes and types of documents.
As a document is transported along its process path through the system, the document's precise position must be known and controlled. The adjustment of a document's position is generally controlled via a registration process and apparatus. Registration systems can comprise nip rolls in combination with document position sensors whereby the position sensors provide feedback control of the nip rolls to adjust the document to the desired position.
Regardless of the registration system employed to control the position of a document for subsequent printing, misregistration of images printed on a document can occur when multiple printing modules mark an image on a document or media sheet.
One example is a duplex printing operation utilizing two printing modules, whereby printed pages will be bound such that facing pages are printed using two printing modules. This situation occurs when the first side of all documents is printed with one printing module and the second side of all documents is printed with a second printing module. After the finished documents are sequentially bound, page two of the first document will face page one of the second document. Small misregistration of the printed images can become noticeable to a viewer due to registration inconsistencies between the printing modules and other hardware associated with the registration of the document prior to printing.
To eliminate small misregistration inconsistencies of printed images between printing modules, as described above, it is desirable to print the facing pages of a booklet-type bound collection of documents utilizing the same printing module. Usually, this involves an inverter placed at the output of a printing module before releasing the media sheet to an output device. The inverter inverts every second sheet, thereby arranging the printed documents at the output device such that facing pages are printed with the same printing module.
In practice, the time needed to invert a sheet is longer than the time needed for the sheet to simply bypass the inverter. Consequently, as a printed document is inverted, the subsequent printed documents, which will not be inverted, must be delayed in time to prevent them from advancing relative to the inverted document and crashing into it. The lower productivity associated with this delay is undesirable since it represents unused printing module capability. Some systems reduce the sheet delay by displacing the inverter further from the printing module, thus allowing sheets to speed up before entry to the inverter which reduces the cycle time of inverting a document. However, some print system architectures preclude this approach.
What is needed is a media sheet inverter apparatus and method to reduce timing delays associated with the operation of a duplex printing system as generally described above.